1. Field of the Invention
The present invention relates to an inkjet recording apparatus that includes a recording head capable of discharging ink on a recording medium.
2. Description of the Related Art
A recording apparatus includes a recording head shifting in a direction perpendicular to a conveying direction of a recording medium (e.g., a paper). The recording head for a recording apparatus is a wire-dot type, a thermal type, a heat transfer type, or an inkjet type.
An inkjet recording head does not cause frictions with a recording medium due to its non-contact nature and is capable of realizing high-speed printing. An ink droplet, discharged from a recording head, impacts on a recording medium spaced from the recording head. The inkjet recording head has low noise in a recording operation and can effectively output color inks at a low running cost.
However, depending on a positional relationship between a recording head and a carriage, or due to a tolerance in a process of forming ink discharge ports on a recording head, an array of ink discharge ports may incline relative to a line feed (LF) direction (i.e., a recording medium conveying direction). If a recording head having an inclined ink discharge port array is used to record a vertical rule, a recorded rule inclines relative to an ideal line.
An image recording apparatus including a plurality of discharge ports disposed in parallel can discharge different color inks from respective discharge port arrays. An image recording apparatus performing color printing may generate image deterioration, such as granular quality deterioration, due to tiny deviation in impact position of discharged ink dots. In particular, output image quality tends to greatly deteriorate in a multi-pass printing operation performing forward/backward printing operations.
A conventional correction method applicable to a recording apparatus includes obtaining the inclination of a discharge port (nozzle) array and shifting the ink discharging timing so as to adjust a dot impact position to an ideal state in a main scanning direction.
A conventional inclination correcting method discussed in U.S. Pat. No. 5,956,055 is applicable to a recording apparatus that performs a first scanning operation using a lower nozzle section for dot recording, a recording paper feeding operation, and a second scanning operation using an upper nozzle section for dot recording. The correction method discussed in this prior art includes obtaining a deviation amount (i.e., head inclination) of the dot position (impact position) in the main scanning direction based on the recorded dots, determining an offset amount of the recording position, and shifting at least part of the nozzles by a distance dependent on the determined offset amount.
Hereinafter, the inclination correcting method discussed in the U.S. Pat. No. 5,956,055 will be described in detail with reference to FIGS. 31 and 32. FIG. 31 illustartes a printing result of a vertical rule formed by a nozzle array inclined by a deviation amount equal to 2 dots/1200 dpi at front/rear edges of the nozzle array relative to the main scanning direction.
FIG. 32 illustartes a printing result of a vertical rule which is subjected to the conventional inclination correction discussed in U.S. Pat. No. 5,956,055. The correction according to this prior art includes shifting a nozzle division 3202 in the main scanning direction by an amount equal to 1 dot/1200 dpi without shifting a nozzle division 3201 positioned at an upper portion of an inclined nozzle array. More specifically, the drive timing for the nozzle division 3202 is changed by an amount equal to 1 dot/1200 dpi while the drive timing for the nozzle division 3201 is unchanged.
As apparent from the comparison between FIGS. 31 and 32, a width 3200 representing an impact deviation amount of a corrected vertical rule in the main scanning direction is shorter than a width 3100 representing an impact deviation amount of a non-corrected vertical rule (i.e., width 3200<width 3100). Thus, it is understood that the inclination correction can obtain a corrected vertical rule resembling an ideal output state.
As discussed in Japanese Patent Application Laid-Open No. 07-309007, a conventional inkjet printing system includes an error correcting circuit capable of visually reducing a rotational error of a recording head with an offset added to a nozzle for correcting a recorded image.
As discussed in Japanese Patent Application Laid-Open No. 07-40551, a conventional inkjet recording apparatus changes the order of driven blocks and also changes the interval of the blocks according to the inclination. Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2004-9489, a conventional inkjet recording apparatus includes a unit capable of changing data allocated to a nozzle based on the inclination of a head.
The above-described conventional inclination correcting methods have the following problems.
First problem: Band unevenness generated by bidirectional misregistration
The above-described conventional inclination correcting methods can improve the printing quality by reducing an impact position deviation width of a vertical rule in the main scanning direction if a recording head is inclined.
However, according to the above-described conventional inclination correcting methods, no special consideration is made for a printed result of multi-pass printing. The multi-pass printing is characterized in that a recording head performs plural scanning operations in a same region before a final image is formed.
The multi-pass printing can bring optimized effects when a dot impact position in a forward printing operation completely overlaps with a dot impact position in a backward printing operation (the registration is matching in forward/backward directions).
However, in a multi-pass printing performing forward/backward printing operations for realizing a high-speed image output, no consideration is given to a method for correcting band unevenness generated due to slight deviation in the dot impact position between the forward printing operation and the backward printing operation (i.e., misregistration in forward/backward directions).
In general, the misregistration in forward/backward directions tends to be easily generated if the setting by a user or a sensor is performed. Furthermore, users are sensitive to any deterioration on an image and therefore an image, if it deteriorates due to band unevenness, maybe recognized as having an extremely lower quality level. Accordingly, a printing control method capable of suppressing band unevenness generation is required.
Second problem: Costs for realizing higher correction resolution
According to an inclination correcting method depending on correction data, a recording apparatus processes a great amount of data to improve data resolution. For the correction capable of visually eliminating any deviation in the recording position, a recording apparatus processes a great amount of image data and cannot perform a recording operation at a higher speed. The cost for a recording apparatus increases.
Furthermore, according to an inclination correcting method depending on a head driving control, a recording apparatus generates and selects a plurality of drive signals to improve correction resolution. This method also complicates a recording apparatus and increases the cost for the recording apparatus.